Show a loading state in the strategy dropdown

While the strategy list is being fetched the dropdown now shows a disabled
"Loading strategies…" option instead of an empty select, and "No strategies
available" once loading finishes with no results. Applies to the strategy,
backtest and sweep panels via a loading prop. Adds tests.

frontend/src/components/SweepPanel.vue

@@ -7,6 +7,7 @@ import CollapsiblePanel from './CollapsiblePanel.vue'
 const props = defineProps({
   strategies: { type: Array, required: true },
   status: { type: Object, required: true },
+  loading: { type: Boolean, default: false },
 })
 
 const running = ref(false)
@@ -97,7 +98,10 @@ async function run() {
     <div class="grid gap-2 sm:grid-cols-4">
       <label class="space-y-1">
         <span class="label">Strategy</span>
-        <select v-model="selectedKey" class="input" :disabled="running">
+        <select v-model="selectedKey" class="input" :disabled="running || !strategies.length">
+          <option v-if="!strategies.length" value="" disabled>
+            {{ loading ? 'Loading strategies…' : 'No strategies available' }}
+          </option>
           <option v-for="s in strategies" :key="s.key" :value="s.key">{{ s.name }}</option>
         </select>
       </label>